1. Technical Field
The present disclosure relates to a laser unit.
2. Related Art
With miniaturization and high integration of a semiconductor integrated circuit, an improvement in resolution has been demanded for a semiconductor exposure apparatus. Hereinafter, the semiconductor exposure apparatus is simply referred to as an “exposure apparatus”. Shortening in a wavelength of light to be outputted from an exposure light source has been in progress accordingly. A gas laser unit is used in place of an existing mercury lamp for the exposure light source. Currently, a KrF excimer laser unit and an ArF excimer laser unit may be used as gas laser units for exposure. The KrF excimer laser unit may output ultraviolet light with a wavelength of 248 nm, and the ArF excimer laser unit may output ultraviolet light with a wavelength of about 193.4 nm.
As current exposure technology, liquid immersion exposure is practically used. In the liquid immersion exposure, a clearance between a projection lens on exposure apparatus side and a wafer is filled with a liquid to change a refractive index of the clearance, thereby shortening an apparent wavelength of light from the exposure light source. When the liquid immersion exposure is performed with use of the ArF excimer laser unit as the exposure light source, ultraviolet light with a wavelength of 134 nm in water is applied to the wafer. This technology is referred to as “ArF liquid immersion exposure”. The ArF liquid immersion exposure is also referred to as “ArF liquid immersion lithography”.
Since a spectral line width in free oscillation of each of the KrF excimer laser unit and the ArF excimer laser unit is wide, e.g., in a range from about 350 pm to about 400 pm, color aberration of laser light (ultraviolet light) that is reduced and projected on the wafer by the projection lens on the exposure apparatus side occurs, which results in decrease in resolution. It is therefore necessary to narrow a spectral line width of laser light to be outputted from the gas laser unit to an extent in which the color aberration is negligible. The spectral line width is also referred to as “spectral width”. Accordingly, a line narrow module including a line narrowing device is provided in a laser resonator of the gas laser unit, which achieves narrowing of the spectral width. Non-limiting examples of the line narrowing device may include an etalon and a grating. The laser unit narrowed in spectral width in this way is referred to as “line narrowing laser unit”.
Moreover, in recent years, laser annealing has been as one of techniques of crystallizing an amorphous film provided on a glass substrate or a silicon substrate to form a polycrystalline film. Laser annealing may involve, for example, pulse irradiation, with laser light, of an amorphous silicon film provided on a silicon substrate to form a polycrystalline silicon film, with use of a laser annealing apparatus equipped with an excimer laser, etc. Forming a polycrystalline silicon film in this way may allow for formation of thin film transistors. A substrate with thin film transistors formed in this way may be used for liquid crystal displays, etc. A KrF excimer laser unit, a XeCl excimer laser unit, and a XeF excimer laser unit may be used as light sources for the laser annealing apparatus. Laser annealing may be performed by irradiation of an amorphous silicon film with laser light in free oscillation. For example, reference is made to Japanese Unexamined Patent Application Publication No. 2007-012805, Japanese Unexamined Patent Application Publication No. H02-098919, Japanese Unexamined Patent Application Publication No. 2000-150998, Japanese Unexamined Patent Application Publication No. 2008-277616, Japanese Unexamined Patent Application Publication No. 2008-277617, and Japanese Unexamined Patent Application Publication No. 2008-277618.